1. Field of the Invention
The present invention relates to an image forming apparatus, such a digital copying machine, a digital printer, an ink jet printer, and a printer. More particularly, the invention relates to an invisible information recording method for recording an image being different from a visually perceived image on a part of a sheet of a paper on which an image is recorded, and an image forming apparatus using the same. Further, the invention relates to a printing system which easily retrieves the already printed document data from a part of a printed document formed by the image forming apparatus.
2. Description of the Related Art
In the field of the image forming technique, there is known a technique in which another information, such as text and characters, which is different from an image is added to the image for the purpose of increasing the amount of information or securing a secrecy.
Japanese Patent No. 2958396 discloses a technique to embed the additional information in a full line screen or a dot screen. In this technique, the additional information can be written into a location out of the full line or screen. In this connection, it is known that the additional information may be embedded into the location where the full line or screen is not printed if a screen is printed there to such an extent as not to be offensive to the eye. Where a high quality print is required, however, such a thin screen printing is not allowed frequently. Further, it is difficult to read the additional information, which is embedded into the thin screen. Further, where the printed screen that is so thin to be not offensive is used, it is difficult to read the information embedded therein.
Description of embedding the additional information into the image by various ways is found in “Fundamentals of Electronic Watermark” written by Kashio Matsui, published by Morikita Shuppan corporation in Japan. Matsui also describes that in the case of the image having a relatively high redundancy, such as the screen print, various methods to embed the additional information into the image are proposed and practically used. Further, he describes that the additional information may be embedded into the image having a low redundancy, such as a document image, by changing the character pitch, width or inclination.
In recent image forming apparatus, such as a digital copying machine, digital printer, ink jet printer, printer and the like, the resolution is remarkably improved, and is comparable with that in the photo-offset print. Accordingly, the character pitch, width, inclination or the like directly and adversely influences the print quality in the document image. For this reason, it is difficult to embed the additional information to the image by changing the character pitch, width or the inclination. Also in this case, so long as the information about the original character is unknown, it is difficult to read the information that is embedded on the basis of the changing of the character pitch, width or inclination.
Recently, there is proposed a recording method in which the additional information is embedded into the image without deteriorating the print quality in a manner that micro dots are printed on the image in an invisible fashion.
In the recording method, a diameter of each dot is typically 0.1 mm, and the dot of this size is visible. If the amount of the additional information is several bytes, an image containing the information embedded therein does not give the viewer an impression of its smearing by the embedded information. However, if the amount of the additional information embedded is several hundreds bytes or larger, an image containing the additional information embedded therein frequently has a thin black stripe appearing in its location where the additional information is stored. This is problematic where a high quality print is required.
It is well known that when invisible, isolated micro dots are embedded into the image, the recording reliability is deteriorated. The laser printer or the like sometimes suffers from an undesired phenomenon, called fog, in which toner particles attach to a location other than a desired recording area. In this case, it is difficult to distinguish the isolated informative micro dots from scattered toner particles. Also in the case of the ink jet printer, the isolated, micro dots as printed are absorbed into the recording sheet, so that a recording density reduces. Additionally, in this printer, fine particles, called ink satellite, which are generated when ink particles are separated, are present in addition to the ink particles used for the purpose of recording. When those fine particles attach to the recording sheet, it is difficult to distinguish the isolated micro dots from those fine particles attached.
Various techniques to embed additional information into the image have been proposed, as described above. Those proposals have commonly the following problems. As the result of embedding the additional information, the image quality is deteriorated. It is difficult to read out the embedded information. Further, when the micro dots that cannot be perceived are printed, the reliability is unsatisfactory.
The archiving printer, when printing, receives print data and a job ticket defining a print form by a network or another transmission means. The archiving printer prints the print data according to the job ticket. The printer, while printing, stores the print data into a recording medium, called an archive, which is provided in the archiving printer or the controller exclusively used for the archiving printer.
In the subsequent printing by the archiving printer, the user can obtain a desired print by merely issuing the job ticket. In this case, the user must designate a storage position of a document to be printed out in the printer or the archive of the controller. The user as a print user receives a report of a location for storing the document thereat in an easy-to-understand form from the application program. Accordingly, in the case of requesting a re-print, the storage location designation will present a little problem. When many kinds of printed documents, which are similar in version, are present, or when the printed documents are very old and their versions are unknown, a version of a document to be reprinted must be determined by using the information not contained in the printout and the job ticket. In this case, the user must take some measure for this. For example, he or she must prepare information for managing the versions and consume much labor.